The multi-layer ceramic (MLC) technology for fabricating dielectric substrate carriers for integrated circuit/semiconductor package assemblies is well known. Such MLC substrates are produced by preparing a slurry of a suitable ceramic particulates (e.g. alumin), a fugitive resin binder material, a solvent for the resin binder, normally with an included plasticizer; doctor blading the slurry on removable base with subsequent drying to form thin flexible sheets commonly termed ceramic green sheets. The sheets are then punched to form via holes, and the via holes filled with conductive metalliferous (e.g. molybdenum) paste which is also formed into a pattern of conductive lines which will ultimately form the internal circuitry of the MLC. The punched and printed green sheets are selectively stacked on each other into a laminated substrate assembly, as for example, a stack of 15 to 30 green sheets, and the assembly sintered to integrate the sheets and develop the conductor pattern while removing the binder. The resultant fired MLC substrate can then be employed for flip-chip mounting of semiconductor devices which are electrically connected to the internal circuitry of the MLC substrate.
External contact with the MLC substrate is made by a plurality of I/O pins normally brazed or otherwise mounted to the opposite side of the MLC substrate to the internal circuitry thereof. It is desirable that the MLC substrate be formed with vias and lines densified in accordance with the corresponding microminiturization of the integrated circuit developments. Such microminiaturization is desirable in order that the package be compatible with the corresponding densified integrated circuit device chips mounted thereon. In such application the MLC substrate must be provided on the top surface with many small pads which are capable of making registered electrical contact with corresponding closely spaced semiconductor device terminals, e.g. solder mounds. In order to more efficiently use the modern integrated circuit technology, as many as possible integrated circuit devices are supported on and interconnected within the MLC substrate. Such an arrangement keeps the distances between interconnected devices small and thereby minimizes the time it takes for electrical signals to travel between the interrelated devices. Also this reduces the number of electrical connections which must be made thereby reducing the cost of the package and increases reliability. The end result is a highly complex MLC package with a lot of small internal printed circuitry contained in a substrate carrier capable of mounting large numbers of integrated circuit devices.
Further, such MLC substrates require a relatively complex metallurgy on the topside to make electrical connection to integrated circuit devices and to provide engineering change pads, and on the bottom side to make connection to the I/O pads, pins or other type connections. When green ceramic is sintered there is normally a 17 to 20% shrinkage. Frequently, the shrinkage is not uniform throughout the MLC substrate. Since the substrate is relatively large, and the metallurgy geometry quite small, it is difficult and frequently impossible to produce a mask for coating protective diverse metals on the substrate metallurgy, that is 17 to 20% smaller than the original substrate that will have all open areas that will coincide or register with the substrate metallurgy. Such a mask is necessary for depositing additional metallurgy metals using conventional masking techniques. Usually the original metallurgy pattern on the MLC substrate laminate deposited prior to sintering and comprises a refractory (for example molybdenum) metalliferous paste screened on the laminate sheets. After sintering, the refractory metal (e.g. molybdenum, tungsten, etc.) must not only be protectively covered with different metals, as for example nickel, chromium, copper, gold, etc., but also to facilitate the making of connections, as by soldering, to semiconductor devices, compression bonding to wires and/or brazing to I/O pins. The protective coating of the refractive MLC substrate metallurgy, where molybdenum is employed is important, since molybdenum is susceptible to reactive corrosion when exposed to humidity of any pH value at operating potentials common in MLC module or substrate applications. This corrosion interrupts conductors, and leads to module failure.
One method of protecting the substrate metallurgy is by covering them with diverse metals, e.g. nickel, by electroless plating techniques which do not require masks. However, such coatings are usually thin and may contain impurities such as phosphorous boron, etc., which may be objectionable in subsequent joining (e.g. soldering, brazing, compression bonding, etc.).
The protective metal layers can also be deposited by electroplating techniques. However, such plating techniques require electrical connection to each area to be plated, whereas in MLC substrates such connections are not always available, since some pads or portions of the metallurgy pattern can be "electrically floating".
These electroless and electroplating techniques are expensive time consuming and difficult to control; and thus there is a need for a maskless plating technique which is simple with elimination of processing steps as well as having a significant improvement in yield without problems of phosphorous and/or boron contamination.